The present invention relates to a method for communicating information between an S-mode secondary radar and aircraft. More generally, it can be applied to all types of transactions between a transmission/reception system and detected targets.
A transaction between an S-mode secondary radar and an aircraft consists of several electromagnetic pulses representing a selective interrogation sent out by the radar to the aircraft followed by a reception window enabling the reception system of the radar to receive the response sent by the aircraft following the interrogating transmission. The time lag between the interrogation pulse and the reception window depends notably on the distance from the aircraft to the radar. The time taken up by a selective interrogation will range, for example, from 20 .mu.s to 50 .mu.s while the time taken up by a response may be of the order of 85 .mu.s to 136 .mu.s. In a period for the selective interrogation of data elements lasting some milliseconds for example, an S-mode secondary radar generally needs to process several transactions, the aircraft having undergone prior detection and their positions being defined for example by prediction. With these aircraft being detected and their distances from the radar being known, the transactions pertaining to them need to be programmed notably in the above-mentioned given period. However, since the radar antenna is used in transmission and in reception, the interrogation pulses and the windows must not overlap. Since the lengths of the transactions are different and since this overlapping cannot take place, the transactions generally do not fill the periods of time allocated to them in an optimal way because, to prevent overlapping and take account of the different lengths of the transactions, there are varying periods of time between the interrogation pulses or between the reception windows.
Now, air traffic is becoming increasingly dense with peaks of density in certain zones such as airports for example. Consequently, the number of transactions to be processed by S-mode secondary radars is becoming ever greater and these radars are ultimately getting saturated in precise azimuthal sectors with the standard algorithms. An unoptimal use of time by the transactions limits the possibilities of the radars with respect to this increase in air traffic. A more optimized use of time indeed will enable radars to process a greater number of transactions, and hence to correspond with a greater number of aircraft and increase the capacity of S-mode radars.
A known method of programing transactions consists, initially, of classifying the aircraft declared to be active according to their distance, in decreasing order, from the radar, selecting them according to criteria of priority and then positioning the listening windows so that each listening window is placed after ,the one corresponding to the aircraft which is at the immediately greater distance. If this arrangement becomes impossible, namely if the interrogation pulse associated with the last-placed window overlaps a previous window, then the entire transaction constituted by the interrogation pulse and the reception window is placed after the last previous window. This method of programming still leaves substantial periods of time unoccupied and therefore cannot meet the needs of an increase in the number of transactions processed by radar.